In chromatograph-mass spectrometers such as GC-MS and LC-MS, various components contained in a sample are separated by the chromatograph in the time direction, and ions originating from each of the separated components are detected. To quantitate a target component in a sample with a chromatograph-mass spectrometer, generally, a mass chromatogram at a mass/charge ratio m/z corresponding to the target component (also known as an extracted ion chromatogram) is generated, and the surface area of chromatogram peaks occurring in the vicinity of the retention time of the target component in that mass chromatogram is found. Then, referring to a calibration curve prepared in advance, the concentration, i.e. quantitative value, of the target component is computed based on the measured surface area.
For example, in tests for residual agrochemicals in foods and drinks, agrochemicals would be the target components, but such samples contain many interfering components besides the target components, and the target components and interfering components cannot be adequately separated with a chromatograph, so often, the interfering components will affect the chromatogram peaks of the target component. Specifically, if the elution time of an interfering component is close to the elution time of a target component and ions of the same mass/charge ratio (actually, ions included within a predetermined mass/charge ratio range) are produced for both the interfering component and the target component, then peaks originating from the interfering component will overlap peaks originating from the target component on the mass chromatogram at that mass/charge ratio. Furthermore, such peak overlap may cause the retention time for peaks originating from the target component to be recognized incorrectly (offset from where they actually are).
Generally, in order to confirm that a certain peak on a mass chromatogram is indeed a peak originating from a target component, in other words, in order to identify the target component, besides the retention time of the peak on a mass chromatogram at a mass/charge ratio characteristic for the target component, the ratio of signal intensities of peaks at multiple different mass/charge ratios on the mass spectrum obtained at that retention time (the so-called confirmation ion ratio) or the like is used (see Patent literature 1, etc.). However, when the influence of interfering components is great as described above, to ensure reliability of analysis, the operator would in actuality have to visually confirm each mass chromatogram and mass spectrum and determine if the influence of interfering components is or is not present.
In testing for residual agrochemicals in foods in Japan, currently, a positive list system has been adopted, with analysis of over one hundred types of residual agrochemicals being performed at once, but for a very large number analytes, the conventional technique described above is inefficient in that it takes much effort and time for an operator to perform the confirmation operation for all the target components. Thus, as a technique for improving the efficiency of the analysis operation, the method is also used whereby thresholds are established in advance for the quantitative value (concentration) and confirmation ion ratio for each of the target components, these are narrowed down to only those components for which the quantitative value or confirmation ion ratio obtained through actual analysis exceeds the aforementioned threshold, i.e., components which may have been influenced by interfering components, and mass chromatograms and mass spectra are confirmed one by one only for those components.
In terms of specific methods, for the analysis of residual agrochemicals in foods, the method of classifying target components into three groups (G1) through (G3) as follows is known in the prior art.
(G1): Group of components for which confirmation operations are unnecessary because the component is clearly not contained in the analyte in question or because the measured quantitative value is at or below the threshold.
(G2): Group of components which are clearly present at a high concentration because the measured quantitative value exceeds the threshold and the confirmation ion ratio is at or below the threshold.
(G3): Group of components for which it is unclear whether the component is the assumed target component or not and for which additional confirmation is required because the measured quantitative value exceeds the threshold but the confirmation ion ratio also exceeds the threshold.
If classification of components as described above can be performed with high accuracy, it will suffice to confirm the mass chromatogram and mass spectrum only for components classified as (G3), so operator effort and time can be greatly reduced as compared to the case where the same confirmation operation is performed for all the detected components. However, if peaks originating from interfering components overlap peaks on the mass chromatogram at the mass/charge ratio for quantitative calculation (the so-called quantitation ion mass/charge ratio) and the peak area increases, then the quantitative values computed based on peak area will be inflated, and components which should properly be classified as (G1) may end up being erroneously classified as (G3). Furthermore, if it becomes impossible to detect peaks near the location of the target component's retention time due to overlap of large peaks originating from interfering components, or the like, then components which should properly be classified as (G2) or (G3) may end up being erroneously classified as (G1).
Here, examples of cases where erroneous classification is performed will be described based on FIG. 6.
Here, data has been acquired with M1 being specified as the quantitation ion mass/charge ratio and with M2 and M3 being specified as confirmation ion mass/charge ratios for confirmation ion ratio calculation. Furthermore, it will be assumed that the standard mass spectrum corresponding to the target component (the standard mass spectrum) is substantially the same as the measured mass spectrum of data A shown in FIG. 6 at (a). Moreover, there are two conditions for being classified as (G3): (i) the concentration corresponding to the intensity of a peak characteristic of the target component observed on a mass chromatogram at mass/charge ratio M1 (hereinafter, such a mass chromatogram will be denoted as mass chromatogram (M1)) is at or above a reference value (10 ppb), and (ii) the confirmation ion ratio is outside the reference range (at or above a threshold value).
In data A shown in FIG. 6 at (a), the concentration and confirmation ion ratio for the peak originating from the target component designated by the downward facing arrow on mass chromatogram (M1) are calculated without being affected by peaks originating from interfering components appearing on either side of that peak. Thus, the concentration exceeds the reference value and the confirmation ion ratio is within the reference range, so the target component of interest is clearly present and is thus excluded from the narrowed set. In this case, the target component is classified as (G2).
On the other hand, in data B shown in FIG. 6 at (b), the peak originating from the target component on the mass chromatogram (M1) is overlapped by the large peak originating from an interfering component present immediately before it. Thus, identification of the target component is judged not based on the original retention time but on a peak originating from a different component with an offset retention time as shown by the downward facing arrow. In this case, the computed concentration is 5 ppb, which is lower than the actual concentration (15 ppb) of the target component and is below the reference value, so the target component in question is erroneously deemed to not require confirmation and is excluded from the narrowed set. In this case, the target component is classified as (G1).
In data C shown in FIG. 6 at (c), although the target component of interest is not contained, a peak originating from a different component with a similar retention time is detected on the mass chromatogram (M1), and identification of the target component is performed using the concentration and confirmation ion ratio based on that peak. As a result, the target component is classified as (G3) and is included in the narrowed set even though there is clearly no need for such inclusion to begin with.
Moreover, in data D shown in FIG. 6 at (d), the retention time of the target component of interest and of one interfering component are substantially the same, so this interfering component inflates the intensity of the peak originating from the target component. As a result, the computed concentration becomes a concentration higher than the actual concentration (5 ppb) of the target component, and as a result, the target component is classified as (G3) and is included within the narrowed set even through there is clearly no need for such inclusion to begin with.
In this way, in the above examples, based on the actual concentration of the target component, both data A and data B should be included in the narrowed set, but data B ends up being excluded, and conversely, data C and data D should by nature be excluded from the narrowed set, but end up being included in the narrowed set. Namely, when evaluating whether or not the component is to be included in the narrowed set based on actually measured data, false negatives and false positives occur, leading to a reduction in screening accuracy and to unnecessary effort being expended on the screening.
If a mass/charge ratio not affected by interfering components could be selected as the quantitation ion mass/charge ratio, it would be possible to avoid the problems described above. However, the interfering components which are present differ depending on the type of food which is the analyte, the method of pretreatment for analysis, the type of liquid phase used in liquid chromatography, and the like. Thus, it is practically rather difficult to select a mass/charge ratio not affected by interfering components in advance as the quantitation ion mass/charge ratio.